1. Field of the Invention
This invention relates generally to the fabrication of microelectronic devices, such as MRAM (magnetic random access memory) or TMR (tunneling magnetoresistive) devices, and relates particularly to a photolithographic process using suspension-bridge shaped masks for the formation of small, isolated device features.
2. Description of the Related Art
The formation of small individual device features within microelectronic fabrications generally makes use of photolithographic patterning. Such patterning can be subtractive or additive. Subtractive patterning involves the deposition of a large structure upon a pre-existing fabrication, followed by the removal of portions of that structure to leave behind the smaller feature which is required. Additive patterning involves the formation of the small feature directly upon the pre-existing fabrication. Either approach is implemented by the formation of a photolithographic mask, which is usually a bi-layer mask called a lift-off mask. Such a lift-off mask, whose use is well known in the art and which is shown schematically in FIG. 1a when used in a subtractive process, is formed in two layers: a relatively wide upper portion (6) resting upon a narrower lower pedestal portion (4) which is undercut (8). The edges of the upper portion are symmetrically disposed about the pedestal. The mask can be used as an etching mask to remove portions (10) (shown in broken lines) of a device layer (2) (such as a layer of conducting material or a magnetically active layer) which is formed on a substrate (1). A portion (12), of a desired shape, remains. Typically, as shown in FIG. 1b, the removed portions ((10) in FIG. 1a) are then filled in with a dielectric material (16), now using the mask as a deposition mask, and the mask (covered with residue (20)) is subsequently lifted off by dissolving its pedestal. As shown schematically in FIG. 1c, the portion of the layer left behind (12) after mask removal, is the desired small feature and it is now surrounded by an insulating dielectric (16). The entire surface may now be planarized for additional process steps.
The use of lift-off masks for forming small device features has certain disadvantages, which are a result of the difficulties in forming small, uniformly undercut pedestal layers. As shown in FIG. 1d, the non-uniformity of these layers due to undercut control problems can lead to either the buildup of deposited material (24), called “fencing,” around the edges of the mask (if the undercut is insufficient) or, as shown in FIG. 1e, to the collapse of the mask entirely (if the undercut is too extreme). These difficulties and a method of eliminating them have been disclosed by Han et al. (U.S. Pat. No. 6,493,926 B2), which is assigned to the same assignee as the present application and which is fully incorporated herein by reference. Han discloses a the formation and use of a novel suspension-bridge shaped mask, whereby a small giant magnetoresistive (GMR) sensor is formed within an opening beneath a suspended overhanging region, thereby eliminating the problems associated with a non-uniform pedestal if the GMR sensor were formed using the prior-art bi-layer stencil. The present invention extends the suspension-bridge type stencil to the formation of small isolated device structures required in the fabrication of MRAM devices and TMR structures. To achieve the advantageous results of the suspension-bridge type mask in the present circumstances, it is required that two overlapping suspension-bridge type masks be used in a novel sequence and mask configuration.
Small TMR device structures and methods for forming them are disclosed by Shimazawa et al. (U.S. Pat. No. 6,562,199 B2) and by Saito et al. (U.S. Pat. No. 6,522,573 B2). Neither of these inventions include the use of single or multiple suspension-bridge type photolithographic masks in forming the small device structures.